As is well-known, certain gases can be measured both qualitatively and quantitatively by means of infrared radiation absorption wherein a detector cell, typically containing a quantity of the same gas as that desired to be measured, utilizes a sensor to measure thermal expansion of the gas within the detector cell induced by the absorption of the infrared radiation. The gas to be analyzed is typically contained within a sample cell such that it attenuates the infrared radiation prior to the infrared radiation entering the detector cell. The particular wavelength and amount of infrared radiation absorbed by the gas within the detector cell provides an indication of the type of gas present in the sample cell and the quantity thereof, since such absorption is a function of the attenuation occurring in the sample cell.
Numerous prior art infrared gas detection devices have been developed which are generally known as pneumatic detector analyzers. Such prior art pneumatic analyzers have the ability to analyze a gas of interest in the presence of other interfering gases having overlapping infrared spectrums. In such pneumatic analyzers, the selectivity of the analyzer is attained by using the gas to be analyzed in the detector to make the detector sensitive to the same.
The vast majority of the early prior art non-dispersive infrared analyzer devices employing pneumatic detection techniques utilized two separate radiation beams wherein the unknown gas sample was passed through the first beam and a second beam as employed to produce a balancing effect. Examples of such dual-beam systems are found in U.S. Pat. No. 2,573,870, issued to Pfund and U.S. Pat. No. 2,698,390, issued to Liston, one of the co-inventors of the subject application.
Such dual-beam detection techniques possessed the advantage of reducing the adverse effects of encountering small changes in source emissions, detector sensitivity, and amplifier gain and produced a signal which had a larger effective change due to the presence of the gas desired to be analyzed- However, the compensation for source emission changes was not sufficient in such prior art devices due to the dual beams not having identical views of the radiation source. Further, changes in the optical transmission of the dual beams also produced an undesirable signal drift; for example, a very slight accumulation of dirt or debris in the sample cell of such prior art dual beam systems would produce a significant zero shift in the detector signal.
Subsequently, pneumatic detector infrared gas analyzers were developed employing a single radiation beam utilizing either dual detectors or dual radiation sources. Example of such single-beam pneumatic analyzers utilizing dual detectors are found in U.S. Pat. No. 3,105,147, issued to Weilbach et al. and U.S. Pat. No. 3,130,302, issued to Liston et al, while an example of single-beam pneumatic analyzers utilizing dual radiation sources is found in U.S. Pat. No. 3,227,873, additionally issued to Liston. Such prior art single-beam instruments had the advantage that the radiation source was viewed through an identical optical path by the detectors with the second detector of the dual detectors additionally affording a means to compensate for interference from other compounds found in the gas sample. The prior art single-beam dual detector instruments had the advantage that compensation for changes in the source radiation and/or sample cell was excellent. However, such prior art analyzers had the significant disadvantage that differences in temperature coefficients of the dual sensors produced a significant signal drift. Further, such dual sensor devices proved to be significantly expensive to produce.
Most recently, infrared gas analyzers employing a pneumatic detection technique have been developed employing a dual chamber formed in optical series with a single sensing element or detector. The use of such optical series detector chambers provide a common optical path which reduces the effects in transmission of the sample cell and source emission. It also reduces the effect of changes in the detector sensor sensitivity. However, this compensation is incomplete since the signal from the front detector chamber is always stronger. This difference in signal was due to the fact that the infrared radiation was substantially attenuated by the gas contained within the front detection cell and thus did not affect the rear detection cell to the same degree.
By balancing the signals from the front and rear detector chambers these adverse effects are eliminated since a change in the energy entering the detector does not change the signal output. The closer to balance the less the effect of these variables.
The adverse effect of signal strength difference in the front and rear chambers is of particular concern in prior art infrared analyzers utilizing the mass flow of gas as a means of measuring infrared radiation absorption. In such prior art infrared analyzers, gas flow through a conduit interconnecting the front and rear detection cells is measured to provide an indication of infrared absorption in the two detection cells. If the front detection cell's optical path is reduced in an effort to achieve balance, then the volume that is available for the gas flow from the second detection cell is restricted by the buildup of pressure in the first detection cell, thus reducing the signal from the second detection cell and defeating the attempt to balance the signals.
Thus, there exists a substantial need in the art to provide an optically seriesed detector infrared analyzer in which the signals from the front and rear detection cells are balanced and which does not suffer from restriction of gas flow due to the buildup of pressure in the first detection cell.
Additionally, N.sub.2 has been typically employed as a diluent gas in contemporary pneumatic detector infrared analyzers. The diluent gas is that gas which is mixed with the gas of interest and generally used to charge at least one of the two detector cells. Since it is the gas expansion or flow produced by the selective absorption of infrared radiation by the sensitizing gas which is measured by the sensor, it is thus desirable to minimize any counteracting effects of the diluent gas. The thermal conduction of the diluent gas determines the rate at which heat absorbed by the sensitizing gas is conducted to the walls of the cell. Thus, there exists a need in the art to use a diluent gas having minimal thermal conduction.
Furthermore, when employing a mass flow sensor, a diluent gas with low specific heat gives a much larger signal for the same amount of thermal energy. Therefore, it would similarly be desirable to utilize a diluent gas having the lowest specific heat.
In an analyzer where operation is near or at balance, synchronous detection of the detector signal is required to avoid confusion as to which side of balance the system is. High accuracy and resolution is required to prevent degrading the system. Previous analyzers have employed synchronous switching of the analog signal for this purpose. A high resolution analog-to-digital converter is required to convert this rectified signal for processing by modern microprocessors. In the invention a voltage-to-frequency converter circuit is employed using the counter in the microprocessor for this purpose. This voltage-to-frequency circuit is much cheaper and more accurate.